J .O. Evjemo, Y. Olsen J. Exp. Mar. Biol. Ecol. 242 1999 273 –296
287
21 21
Fig. 6. A Relationship between absolute production rate mg C ind day
and animal dry weight and B
21
relative production rate day and animal dry weight of A
. franciscana fed different concentrations I. galbana.
4. Discussion
4.1. Growth of A. franciscana at different food concentrations When filter feeders such as A
. franciscana are maintained under conditions of excess food they might attain maximum growth rate which is believed to be determined by
21
temperature Miller et al., 1977, animal density ,2 ind ml , Abreu-Grobois et al.,
3 21
1991, food concentration .41–233?10 cells ml
, Nimura, 1980, physical dis- turbance and water quality Coutteau, 1992. For most species of the Artemiidae and
different Artemia-strains the optimum temperature is in the range of 25–308C Sorgeloos et al., 1986. This means that A
. franciscana, feeding on maximum food concentrations of I
. galbana under laboratory conditions, most probably, grew at a rate
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.O. Evjemo, Y. Olsen J. Exp. Mar. Biol. Ecol. 242 1999 273 –296
Fig. 7. Yield of 1–12 days old A . franciscana grown at five different concentrations of I. galbana plotted as a
function of animal dry weight.
21
close to its maximum growth rate, reaching a dry weight of 180–194 mg ind postlarval stages I–III, 5.9 mm at day 8 final density of animals 0.48–0.81 ind
21
ml . The digestibility and the nutritional value of the algae will, to some extent also
affect the growth potential of the animals, but this has not been further evaluated in this study. Sick 1976 has shown that A
. franciscana grown on five different species of marine algae, at a temperature of 258C, showed significantly different survival, growth
rate and assimilation rate. Dunaliella viridis gave highest growth rate of A . franciscana,
but these values are 10–12 lower compared with the values obtained for I . galbana,
and might be related to different diets and different temperatures. The animal density
21
ind ml could have reduced the growth rate during the first days of the growth period
21
because initial density was 16–18 ind ml . From incubation until day 4 the two highest
food concentrations were reduced by 4–8, and in order to keep the food concentrations
21
as stable as possible the animal densities were diluted down to 1.1–2.4 ind ml day 4
21
reaching 0.48–0.81 ind ml at day 12. In this period day 4–12 animals were sampled
from the cultures every 24 h due to dry weight, carbon nitrogen analysis and for measurements of the ingestion rate in the feeding experiment with labelled algae.
Animal density was also evaluated according to measurements on clearance rate of A .
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289
21 21
franciscana [ml ind h
; Evjemo et al. 1999], and kept at a low level in order to avoid fluctuations in the food concentrations. Since the food concentrations were not
reduced more than 2 between each feeding every 12th hour the animal density was not further reduced during the rest of the growth period. Other workers have reported
21
similar densities of animals. At an original stocking density of 1–2 ind ml , 9.4–9.8
mm Artemia were obtained after 10 days cultivation when feeding on Dunaliella tertiolecta Abreu-Grobois et al., 1991, and in xenic cultures dominated by Chlorella
21
sp. Artemia reached a mean size of 7.0 mm after 9 days at a density of 0.34 ind ml Rosowski, 1989.
The weight versus age curve for A . franciscana fed at the maximum food con-
centrations described adequately the growth of the animals from newly hatched nauplii, during different nauplius stages until post-larval stages I–III which, according to
Schrehardt 1987, is characterised by the development of antenna and genital structures. At day 9 more than 72 of the animals had developed thoracopods X and XI with
filter-seta which is a size-specific criterion for the post-larval stage Schrehardt, 1987, and at day 10 the number of A
. franciscana at this stage had increased to .83. The same curve Fig. 1 also indicates that the growth of A
. franciscana was more influenced by the food concentration after day 4 when the animals had reached a length .1.34 mm
and started the development of the thoracopods, which gradually contribute more in the feeding process Schrehardt, 1987. These morphological changes make the animals
gradually more efficient filter feeders with a better capacity to collect food particles and an increased ingestion rate Fig. 4, leading to a higher growth rate. At all food
concentrations an optimum in the specific growth rate was found when the size of the
21
animals was in the range of 9–20 mg ind , and the differences in specific growth rate
between animals grown at different food concentrations became more pronounced after
21
day 3, when the animals had reached a dry weight .4 mg ind Fig. 3. All these
results indicate a close correlation between development stage and the growth rate of the animals, and shows that maximum growth of the animals was obtained only at food
21
concentrations 10 mg C l maximum food concentrations. At the lower con-
centrations, the growth rate decreased as a function of the availability of food. Nevertheless, Coutteau 1992 found, when feeding Artemia on yeast, that maximal
ingestion rate showed a sharp increase at the early life stages. As the animal size increased the maximum ingestion rate was reached at a gradually lower algal density.
The growth curve Fig. 1 also indicate that the growth of the animals fed at the highest food concentrations decreased when pre-adult stage was reached length 5.2–5.8 mm;
21 21
186–210 mg ind . It is well known that A
. franciscana reach 800–900 mg ind length .10 mm after less than 15 days Rosowski, 1989; Abreu-Grobois et al., 1991.
Recent findings Evjemo, unpublished results have shown that A . franciscana follows
approximately the same growth curve as shown in Fig. 1, from day 1 until day 10, when grown under the same conditions maximum food concentrations, and reach sexual
21
maturity at a length of 10.4–11.6 mm dry weight 890–1070 mg ind after 16 days.
21
As shown in this study the specific growth rate m, day
decreased from approximate- ly 0.4–0.5 at a dry weight of 100 mg day 7 to 0.1–0.2 at day 16. In that study the
results also showed that the growth rate decreased when pre-adult stage was reached, but the growth curves of the animals in Fig. 1 fed the maximum food concentrations, are
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.O. Evjemo, Y. Olsen J. Exp. Mar. Biol. Ecol. 242 1999 273 –296
lower than expected from day 10 until day 12. This might be related to the temperature, nutritional value of the algae or the physiological status of the animals.
21
The animals given 0.2 mg C l did not grow, and the specific growth rate was found
21
to be negative 20.18 day , indicating that the animals were starving. At the higher
21 21
concentration 3 mg C l the animals had a growth rate .0.2 day
animals in the
21
range of 2.1–18.2 mg ind , indicating that the maintenance requirement of food for A
. franciscana, i.e., the food concentration were the animals neither gain nor loose weight,
21
is in the range of 0.2–3 mg C l when grown on I
. galbana. This might be different when the size of the animals increase. As shown in Fig. 1 the animals grown at food
21
concentration 3 mg C l gradually increased the dry weight but the specific growth rate
decreased and did not reach an optimum as in the other cultures Fig. 3. On the other hand the yield Fig. 7 was higher for the animals in this culture and might indicate an
increased utilisation of the food at low concentrations when animal size increases. For
21
other planktonic crustacean like copepods and Daphnia sp., 0.2 mg C l is a
concentration of food, which may sustain both maximum ingestion and growth rate. For
21
instance copepods can grow and reproduce at concentrations ,0.05 mg C l Harris
¨ and Paffenhofer, 1976; Vidal, 1980; Williamson et al., 1985; Piyasiri, 1985 and
Daphniids appear to have consistently higher values of the incipient limiting con-
21
centration than copepods, 0.2–0.4 mg C l Geller, 1975; Porter et al., 1982; Taylor,
1985; Bohrer and Lampert, 1988. 4.2. Carbon and nitrogen content
The carbon content of A . franciscana of dry weight was found to be quite stable
Fig. 2, and independent of the growth conditions 36.2–45 of dry weight. Oppenheimer and Moreira 1980 have, on the other hand, found striking differences in
the carbon content for different developmental stages of Artemia 27.5 to 55.6 of dry weight, using a strain from San Francisco, CA, USA. In both studies, however, the
variation was most pronounced when the nauplii molts from instar I into instar II and Oppenheimer and Moreira 1980 relates these changes to the period of ‘self-absorp-
tion’, when the nauplii change from endogenous metabolism of the yolk reserve nauplius, to actively filtering food particles from the food suspension metanauplius I.
The variation in nitrogen content ranged between 8.4 and 10.5 in both studies, but an initial drop in the nitrogen content, as was found for the carbon content, was only seen
in this study Table 2.
4.3. Ingestion rate A. franciscana The ingestion rate increased with increasing food concentration and size of the
21
animals. At the highest food concentrations 10 and 20 mg C l no significant
difference was obtained P ,0.001 Fig. 4, and only the size of the animals affected the ingestion rate. Compared with the ingestion rate at the lower concentrations 7 mg
21
C l all developmental stages of A
. franciscana had a significantly lower ingestion rate P ,0.001 Fig. 4. At these concentrations both the animal size and the food
J .O. Evjemo, Y. Olsen J. Exp. Mar. Biol. Ecol. 242 1999 273 –296
291
concentration affected the ingestion rate Fig. 4. These results are in close agreement with the Type 3 functional response curves, presented by Evjemo et al. 1999, where
the ingestion rate is plotted as a function of food concentration and follows a sigmoidal curve showing maximum and constant ingestion rate at food concentrations 10 mg C
21
l . Those findings are well reflected in the data presented in Fig. 4 and in the equation
2C
I 5 h998 1 1 270e jL 2 0.91, describing the relationship between ingestion rate I ,
food concentration C and animal size L, mm. The equation gave very close correlation between the predicted and the measured values Fig. 4.
For several species of filter feeding zooplankton a critical food concentration has been detected, often referred to as the incipient limiting concentration ILC Rigler, 1961.
Above this concentration the animals ingest food at a maximum rate and the food concentrations are said to be saturated Mullin et al., 1975, corresponding to the
maximum food concentrations in this study. Below the ILC the ingestion rate decreases with decreasing food concentration McMahon and Rigler, 1963; McMahon, 1965. The
incipient limiting concentration for different stages of A
. franciscana, when feeding on I. galbana, is in the range of 5–7 for metanauplius stages L 5 1.1 mm increasing to
21
9–9.5 mg C l for the post-larval stages L 5 5.2 mm Evjemo et al., 1999. Those
findings support the results obtained in this study were both the growth rate and ingestion rate Figs. 1, 3 and 4 were lower for animals feeding on food concentrations
21
7 mg C l , compared to the animals feeding on the maximum food concentrations
21
10 mg C l . At day 9 the developmental stages of the animals grown at these two
21
food concentrations 7 and 10 mg C l were significantly different P ,0.001. From a
sample of 266 individuals less than 13 had reached post larval stage culture grown at
21 21
7 mg C l whereas more than 88 of the animals grown at 10 mg C l
had reached
21
post larval stage n 5293. All these findings support that 7 mg C l is below the
maximum food concentrations when A . franciscana is feeding on I. galbana.
The weight specific ingestion rate I , Fig. 6 also revealed that the animals grown at
w
the maximum food concentrations ingested the highest amounts of food, and showed maximum values corresponding to a feed intake equal to 20–28 of individual carbon
content per hour. This means that considerably amounts of food were passing through the animal’s gut virtually undigested. Observations from fluorescence microscope
showed high amounts of undigested algal cells in the faecal pellets from animals feeding
21
at concentrations 10 mg C l . The high ingestion rate could be related to the
digestibility of the algae. Coutteau 1992 has shown that the digestibility of food particles affects both the ingestion rate and the assimilation efficiency. When feeding A
. franciscana with different concentrations of a diet having high digestibility the ingestion
rate was lower and the assimilation rate was higher, and nearly constant, compared to the animals giving a diet with low digestibility. In the latter assimilation efficiency
decreased considerably as a function of food concentration. Similar findings have been demonstrated by Evjemo et al. 1999 showing that the assimilation efficiency for A
. franciscana
, feeding on I. galbana, decrease when food concentrations increase a reduction of the assimilation efficiency from 88 to 38, at food concentrations ranging
21
from 0.5 to 30 mg C l . To compensate for the low density of food particles a higher
fraction of the ingested food is probably conversed into biomass at low concentrations
21
3–7 mg C l , and this might explain why the yield Fig. 7 was higher for the
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.O. Evjemo, Y. Olsen J. Exp. Mar. Biol. Ecol. 242 1999 273 –296
animals feeding on low concentrations compared to the maximum food concentrations
21
when animal size ,40 mg ind . At the maximum food concentrations the ingestion
rate was considerably higher Fig. 4 and only a minor fraction of the ingested food must be assimilated from the gut in order to maintain a high growth rate.
The production rate was affected by the food concentration only at concentration ,10
21 21
mg C l and when animal size was .7 mg ind
. We have not considered losses due to moulting, respiratory losses or other forms of excreted carbon that affect the production
rate and the data in Fig. 6 might be slightly underestimated. Production rates reported for the Cladoceran varies with environmental conditions, but production rates .70 are
reported by Kryutchkowa and Sladecek 1969 and Richman 1958. In this study the
21
relative production rate , day reached an optimum 61–68 when the animals
21
size was in the range of 20–148 mg ind . Animals grown at the lowest food
21
concentration had the lowest production rate animals in the range of 7–30 mg ind ,
and to compensate for the low concentration of food particles, the animals utilised probably a higher amount of the ingested food for growth, giving a higher yield Fig. 7.
The results from this study indicate that food concentration is a very important ecological parameter for the growth and production rate of A
. franciscana. According to literature this is well established for both cladoceran and copepods, like in the papers
from Lampert, 1977a, Porter et al., 1982, Mullin, 1963, Mullin and Brooks, 1976, Miller et al., 1977 and Vidal, 1980. Regarding the food concentration for filter feeding
zooplankton, one must consider that in the ocean, in fjords and in many lakes and ponds
21 21
the food concentration mg C l are normally in the range of 0.1 to 1.0 mg C l
Wetzel, 1983; Sakshaug and Olsen, 1986. In the natural habitat of A . franciscana the
6 21
food concentration is considerably higher, reaching 250?10 cells l of Dunaliella sp.
Stephens and Gillespie, 1976, and A . franciscana are probably adapted to relatively
6 21
high food concentrations. For I . galbana the number of cells, 250?10
cells l ,
21
corresponds to 2.6 mg C l Reitan et al., 1993, but the number of cells per volume of
these two algal species cannot be directly compared since Dunaliella sp. is a considerably larger cell Throndsen, 1997, with a greater cell volume. This will lead to
21
a higher food concentration mg C l of Dunaliella sp. than the same cell number per
volume of I . galbana, and might indicate that A. franciscana is adapted to higher food
concentration compared to other planktonic crustacean. Adaptations to different food concentrations is probably the reason why there is a
relatively large difference in the food concentrations where cladoceran and copepods can grow, compared to the results from our study on A
. franciscana. It is also interesting to notice both from our findings and other studies Brune and Anderson, 1989; Rosowski,
1989; Abreu-Grobois et al., 1991; Nimura et al., 1994 that it seems as if the Artemiidae also are adapted to a short growth cycle, with high growth rate in a short period of time,
if the availability of food is at a relatively high level.
5. Conclusions
